On “Air Vortexes”

The media stumbles over a basic aerodynamic aspect of helicopter flight.

I was on Twitter Thursday evening when manp, one of my Twitter friends, tweeted:

So, what is this ‘vortex’ condition with ‘higher than expected temperatures’??? @mlanger any idea?

To be honest, I had no clue what he was talking about. But I Googled “vortex condition with higher than expected temperatures” (don’t you love Google?) and saw an article about the helicopter that went down during the Bin Laden assault in Pakistan. Moments later, manp sent me a link to a Bloomberg article titled “Helicopter Carrying SEALs Downed by Vortex, Not Mechanical Flaw or Gunfire.” The first paragraph read as follows:

A United Technologies Corp. (UTX) Black Hawk helicopter carrying U.S. Navy SEALs to Osama Bin Laden’s hideout was downed by an air vortex caused by unexpectedly warm air and the effect of a high wall surrounding the compound, not mechanical failure or gunfire, according to U.S. officials and a lawmaker.

Whoa. What a mishmash of information. You have to read further into the article where the phenomena they’re trying to explain — vortex ring state — is explained at least two more times by people who actually have a clue what it is. But that first paragraph sure is misleading. It makes it seem as if there was come kind of weird warm air vortex in the compound that brought the helicopter down.

Any vortexes, however, were caused by the helicopter itself. My educated guess of what happened, based on this article and knowledge of helicopter aerodynamics, is this:

As the helicopter was descending inside the 18-foot walls — a descent that was likely nearly vertical — it encountered a setting with power — or vortex ring state — condition. This occurs when the helicopter settles into its own downwash. This may have been made worse by the change in the flow of air due to those 18-foot walls — as suggested in the article. It may also have been made worse by the outside air temperature being warm.

This image from the FAA’s Rotorcraft Flying Handbook helps illustrated what the vortexes are and how they manifest themselves in a hover far above the ground and close to the ground:

Hover Vortexes

As the Rotorcraft Flying Handbook explains:

Vortex ring state describes an aerodynamic condition where a helicopter may be in a vertical descent with up to maximum power applied, and little or no cyclic authority. The term “settling with power” comes from the fact that helicopter keeps settling even though full engine power is applied.

In a normal out-of-ground-effect hover, the helicopter is able to remain stationary by propelling a large mass of air down through the main rotor. Some of the air is recirculated near the tips of the blades, curling up from the bottom of the rotor system and rejoining the air entering the rotor from the top. This phenomenon is common to all airfoils and is known as tip vortices. Tip vortices consume engine power but produce no useful lift. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in accelerating the air in a doughnut pattern around the rotor.

Vortex Ring StateIn addition, the helicopter may descend at a rate that exceeds the normal downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disc. This produces a secondary vortex ring in addition to the normal tip-vortices. The secondary vortex ring is generated about the point on the blade where the airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disc. Rotor efficiency is lost even though power is still being supplied from the engine.

There are three ways to recover from settling with power once you’re in it:

  • Cut power – you can’t settle with power if you don’t have power. This is usually not a good option when you’re very close to the ground.
  • Lower the collective – this reduces the blade pitch. This is also not a good idea close to the ground, since it will result in a descent.
  • Get some lateral airspeed – this breaks you out of the vortex ring state so you’re not settling in your own downwash. This is not possible when you’re surrounded by an 18-foot wall.

(They train us to recover from settling with power using a combination of the second two methods, but we always practice at altitude, since you can get a good descent rate going if you’re really into it. Indeed, settling with power is a serious danger during aerial photo missions requiring hovering at high density altitudes or heavy weights.)

So the pilot did the only thing he could: land hard. Fortunately, although his hard landing damaged the helicopter, it didn’t cause injuries to to men on board. They were able to complete their mission and come home safely. And they left a souvenir lawn ornament in Bin Laden’s yard.

I realize that this is a pretty complex topic and it’s probably not reasonable to expect the press to get it right. But I personally believe that all technical content published in the media should be reviewed by an expert — or at least someone knowledgeable — to make sure it’s not misleading or unclear to the layperson who will read it.

manp is a pilot — although not a helicopter pilot — and he couldn’t figure out what they were talking about. I can only imagine how much that opening paragraph confused the average reader.

6 thoughts on “On “Air Vortexes”

  1. You mentioned that the 18 foot walls may have made things worse. The wall complicates things quite a bit. In the IGE hover diagram above, the downwash does recirculate somewhat, but an awful lot of the recirculating air at the blade tips is free to escape to the sides. If you put an 8 foot wall next to the blade tip, the recirulating air is much more restricted and reduces lift on the wall side of the rotor disk, so much so that the helicopter will be pulled into the wall. It’s a lesson that those of us that have experienced it won’t soon forget. I was able to recover through drastic control inputs, but only because I was aware of the effect and recognized it immediately, and also because I had the power available to overcome the effect. Something that the Blackhawk pilot possibly did not. I say that because early reports stated that the blades did hit the wall.

    • Mike: I think power was a key factor in this incident. He was likely very heavy, carrying troops and equipment. The article mentioned heat and I have to think that had an effect on his available power.

      It’s a very good point you make, though: experience is a better teacher than mere books or lectures. You’d likely not get into the same situation again, since you experienced it once and probably got a good scare that taught a memorable lesson. Similarly, I once got a good scare from an LTE event in a LongRanger and I don’t think that’ll ever happen to me again. We could look at diagrams of vortices every day for the next five years and never have enough information to avoid every possible problem scenario. But if we experience any of them, we’ll remember.

      I think the pilot did a great job, putting it on the ground without anyone getting hurt. I can only imagine the stress of the situation.

  2. On multiple occasions, during the Desert Storm war, I saw helicopter landings where the desert sand became airborne and rendered the vortex effect quite visible. Another image I recall involved landing in the black soot of a burning vehicle, perhaps it was a medivac event, and again the soot made the vortex very evident. I wondered several times since then what goes on in the cockpit upon landing – but not for long because obviously it is well known to pilots what to do upon landing.

    I have once taken off in a small plane behind a larger landing aircraft when there was no wind. That experience taught me to not ignore the profound effects that can result from turbulence, although they manifested quite different from the topic at hand. The proximity of the 18-foot wall certainly made all the difference in this particular hard-landing.

    • Eberhard: I also remember seeing the blade tip vortices during the rain when I worked at the Grand Canyon. I recall sitting up in Papillon’s tower, looking down at the helicopter spinning while parked on the pads below me. The moisture in the air made the vortices quite visible. Very cool to see.

      The turbulence behind and beside large aircraft is referred to as wake turbulence. It can be very bad for helicopters and smaller planes. My first flight instructor told me about a time when his helicopter was almost inverted due to wake turbulence. Dangerous stuff, but easy to avoid if you know how.

  3. Maria, when I was flying a twin BK-117 in EMS we never exceeded 300 feet per minute rate of decent on approach. This was just to avoid settling with power. We also went into an OGE hover at about 1000 feet above ground level in order to check available power before initiating the approach. We not only needed enough power to hover, but enough additional power to arrest the rate of decent before entering the hover on short final. We were required to do a pre landing power check just because there are so many unexpected variables like temperature, humidity, and whether the nurse brought along extra equipment and not telling the pilot. If I found that I just had enough power to barely hover OGE, then I would make my approach at or near zero rate of decent. I wonder if the Blackhawk pilot took the time to do a power check prior to his/her approach or just rely on preflight E6b computations. A mistake not to be repeated. Sometimes we learn the hard way.

    • Mike: It sounds like you followed good, safe practices. That’s one benefit of flying for a company that’s obviously concerned with safety first. I’m sure most pilots don’t do what you do in normal operations — I know I don’t. I suspect this pilot did not take the same precaution to check power settings and make a slow approach — likely because to maintain a certain level of surprise, a quick descent was important. Maybe some lessons were learned by all concerned here.

      I’ve been a passenger on board a helicopter where the pilot came in hot for his landing, ensuring that he reached his spot by doing a quick stop. Later, he asked what I thought of his landing. I told him what I knew to be true: “If you do that with a full load on a hot day at high density altitude, you’re going to land hard.” (I was thinking of those 95°F days I flew at the Grand Canyon, flying a Long Ranger just under max gross weight and doing a confined space landing to the helipads at 6300 feet.) Apparently most instructors are more concerned with “making the spot” than the technique you use to get there. Flying like that will bite you in the butt one day.

What do you think?